(a) Field of the Invention
The present invention relates to a method of orienting liquid crystal materials which are used for the preparation of liquid crystal devices such as liquid crystal optical devices, liquid crystal memory devices, and liquid crystal acoustic devices, to apparatuses to be used for the method, and to a liquid crystal device the liquid crystal material of which is oriented by the method.
(b) Description of the Related Art
One of the known techniques for orienting liquid crystal materials is to use electric field. For instance, (1) R. Simons, et al (Polymer, 27, 811 (1986) discloses a method in which homeotropic orientation is brought about into a liquid-crystalline polymer by disposing the liquid-crystalline polymer between two electrodes and applying an alternating electric field of 60 V, 2 kHz thereto at 150.degree. C. for a long period, (2) in Japanese Patent Application Kokai Koho (Laid-open) No. 63-144324, disclosed is a method in which an insulating layer is provided on at least one of the electrodes supporting a liquid-crystalline polymer (main-chain type or side-chain type) to prevent dielectric breakdown of the liquid-crystalline polymer, thereby enabling application of an electric field sufficient for orienting the liquid-crystalline polymer, (3) in Japanese Patent Application Kokai Koho (Laid-open) No. 63-121815 and Japanese Patent Application Kokai Koho (Laid-open) No. 63-151927, disclosed are methods in which a ferroelectric liquid crystal is cooled slowly after heated until it exhibits isotropic phase, and during the slow cooling, the ferroelectric liquid crystal is oriented by applying an electric field, and (4) in Japanese Patent Application Kokai Kohl (Laid-open) No. 63-243165, disclosed is a method in which a mixture of side-chain liquid-crystalline polymers and low molecular weight liquid crystals is oriented by applying a direct-current voltage.
The method (1) however is poor in productivity because it needs processes for high-temperature heating and for long application of alternative-current voltage. Further, when the method is used for orienting ferroelectric liquid crystals, uniaxially homogeneous orientation, which is essential for ferroelectric liquid crystal devices, cannot be brought about. The reason is that, in cases where the anisotropy of dielectric constant .DELTA..epsilon. of the liquid crystal molecules is positive, homeotropic orientation is brought about, and in cases where .DELTA..epsilon. is negative, although homogeneous orientation can be brought about, the direction of the orientation are randomly varied on the plane parallel to the surfaces of substrates. The method (2) also is to bring about homeotropic orientation into liquid crystals having smectic A phase or nematic phases. Therefore, although the method also can align the liquid crystal molecules parallel to substrates if the liquid crystals have negative anisotropy of dielectric constants .DELTA..epsilon., the direction of the alignment is randomly varied on the plane parallel to the surfaces of substrates, and uniaxially homogeneous orientation cannot be produced. These methods, therefore, have a difficulty that uniaxially homogeneous orientation cannot be brought about into ferroelectric liquid crystals, whether they are low molecular weight liquid crystals or liquid-crystalline polymers. The method (3) needs interfacial treatment, for instance previous coating of substrates with polymers followed by rubbing treatment or previous SiO oblique evaporation, for producing uniaxial orientation, and the electric field applied merely plays an auxiliary role to reduce the infection of the interfacial treatment on the resulting oriented state. Therefore, the process of the method (3) is as complicated as that of conventional orientation methods using rubbing treatment or SiO oblique evaporation, and slow cooling from isotropic phase is essential for the method, resulting in poor productivity. The method (4) needs a specific mixing ratio between side-chain liquid-crystalline polymers and low molecular weight liquid crystals to keep the liquid crystal mixtures in one liquid crystal state at temperatures lower than the liquid crystal phase-isotropic phase transition temperature without causing phase separation and to prevent the change of orientation state at room temperature. The method (4) therefore has a difficulty that it is not always applicable for every liquid crystal mixture.
There are also known methods which use shearing force for orienting liquid crystal materials. An example of such methods is to apply a shearing force to a ferroelectric liquid crystal supported between two substrates by slightly sliding the substrates each other, thereby bringing about homogeneous orientation into the ferroelectric liquid crystal (N. A. Clark. et al.: Appl. Phys. Lett., 36, 899 (1980)). However, the method has a difficulty in producing an oriented state over a large area, and has a problem that accurate adjustment of temperature is required during application of shearing.
With regard to dot matrix liquid crystal devices, there have been proposed various methods for improving their visibility. For example, in addition to conventional black stripe system well known in the art, there have been proposed a liquid crystal display device and a method of producing it in which its orientation film, exclusive of the parts corresponding to the parts of display pattern, is covered with a masking layer, so that its orienting function is taken away from the parts covered with the masking layer, thereby orienting only the parts of liquid crystal corresponding to the parts of display pattern where the orientation film is not covered with the masking layer (Japanese Patent Application Kokai Koho (Laid-open) No. 63-101826). However, the method needs the step of printing the masking layer in order to give orienting function selectively to the parts of display pattern, and, considering the fact that liquid crystal display devices become highly elaborate and highly close increasingly, with the gap separating the display parts from each other narrowed exceedingly, a highly accurate technique is required for printing the masking layer. Providing the orientation film and masking layer itself reduces productivity. Further, the method requires rubbing treatment for orienting the orientation film and, in addition, slow cooling of the liquid crystal device from the temperature at which the liquid crystal exhibits isotropic phase to room temperature whereby the productivity is decreased more and more.